1. Field of the Invention
This invention relates to a process for the preparation of sucrose fatty acid polyesters and the resultant products; and more specifically, to the use of partially etherified sucrose as feedstock in the process. The products are particularly useful as synthetic low calorie fat substitutes for replacing triglyceride fats in food compositions.
The consumption of large amounts of triglyceride fats has been linked to various health problems. For example, one of the most common metabolic problems among people today is obesity. This condition is primarily due to ingestion of a greater number of calories than are expended. Fat is the most concentrated form of energy in the diet, with each gram of fat supplying approximately nine calories, and triglyceride fats constitute about 90% of the total fat consumed in the average diet.
In a U.S. government study, it has been reported that elevation of blood cholesterol levels is a major cause of coronary artery disease, and recommended a reduction in the amount of fat eaten to reduce blood serum cholesterol levels. Thus, there is a need for ways to reduce the amount of triglyceride fats in the diet, in order to reduce the health risks associated with these fats.
2. Discussion of Related Art
Sucrose fatty acid esters are conventionally prepared by transesterifying a lower alkyl ester of higher fatty acids with sucrose. Since sucrose has eight hydroxyl groups per molecule, the number of fatty acid groups bound to sucrose per molecule, commonly referred to as the degree of substitution (D.S.), may vary from 1 to 8. Among them, mono-, di-, and tri-esters find use as non-toxic, biodegradable surfactants and are commercially available in large quantities.
The various known methods for producing sucrose fatty acid esters may be classified into three principal types; i.e., the solvent process, the microemulsion process, and the direct or solvent-free process.
In the solvent process, a fatty acid ester is transesterified with sucrose in a common solvent for the fatty acid ester and sucrose such as dimethylformamide or dimethylsulfoxide in the presence of a basic transesterification catalyst. The reaction may be carried out at a relatively lower temperature, for example, at about 90.degree. C. This process suffers from certain disadvantages in that the solvent used is toxic and, therefore, must be completely removed after the reaction. This is possible in practice only with great difficulty.
In the second process generally known as "microemulsion process", a fatty acid ester is dispersed in a solution of sucrose in a solvent such as propylene glycol or water with the aid of an emulsifier such as alkali metal fatty acid soaps to form a microemulsion, and then the solvent is removed from the emulsion. The reaction is carried out in the absence of solvent and the reaction product does not contain any solvent. Great difficulty is also present in this process for removing the solvent while maintaining the microemulsion state.
In the third process, sucrose is directly reacted with a fatty acid ester by heating their mixture. This process is known as "direct process" or "solvent-free process". Since sucrose and fatty acid esters do not have sufficient affinity to each other, the success of this direct process depends on how well they are contacted in the reaction system. To this end, most of known processes employ an alkali metal fatty acid soap either directly added to or formed in situ in the reaction system to produce a homogeneous molten mixture of reactants.
Consequently, the reaction mixture from the microemulsion process or direct process contains a relatively large amount of alkali metal fatty acid soap, since the soap itself is not a reactant and remains unreacted during the transesterification reaction.
A relatively small amount of alkali metal fatty acid soap is unavoidably formed even in the solvent process by the reaction between the fatty acid ester and the transesterification catalyst such as alkali metal hydroxides and carbonates.
Normally, alkali metal fatty acid soaps remaining in the reaction mixture are separated from sucrose fatty acid esters, while their presence may be tolerated in certain uses such as detergents.
Sucrose fatty acid polyesters may be produced by the following published microemulsion process or solvent-free process.
U.S. Pat. No. 3,963,699 to Rizzi et al. discloses a process for producing sucrose fatty acid polyesters. According to this process, a mixture of sucrose, a fatty acid lower alkyl ester, an alkali metal fatty acid soap and a basic catalyst is heated in the first step to form a homogeneous melt. Thereafter, excess fatty acid lower alkyl esters are added in the second step to the reaction product of the first step. This process suffers from certain disadvantages in that it requires basic transesterification catalysts such as alkali metals, alloys of alkali metals, alkali metal hydrides or alkali metal alkoxides which are expensive and dangerous in handling. The two step reaction is cumbersome in operation and necessarily requires a prolonged reaction time which can lead to the risk of darkening of the reaction mixture.
Generally, sucrose fatty acid esters having a D.S. of greater than 2 are produced by controlling the molar ratio of fatty acid lower alkyl esters to sucrose. Up to a D.S. of 5, polyesters may be prepared at the ratio of fatty acid esters approximately equal to or slightly in excess of theoretical amounts. However, polyesters having a D.S. of greater than 5 require further amounts of fatty acid lower alkyl esters. For example, polyesters having a D.S. of 5.5, 6 and 7 or higher may only be produced at the ratio of fatty acid esters of 6, 8 and 10 moles per mole of sucrose, respectively.
Thus, it is critical for the industrial production of sucrose fatty acid polyesters to minimize the amount of fatty acid lower alkyl esters. The presence of large amounts of fatty acid lower alkyl esters in the reaction system at one time produces certain unique problems. A reaction system containing a large amount of fatty acid esters is less viscous and thus easily susceptible to phase separation which adversely affects the transesterification reaction. Furthermore, relatively large amounts of low boiling point by-products such as methanol are generated and vigorous foaming of reactants takes place during the initial period of the reaction.
Fatty acid lower alkyl esters may be removed from the reaction product by solvent extraction using a solvent such as methanol in which sucrose fatty acid esters are relatively insoluble and fatty acid lower alkyl esters are soluble. However, this technique requires a large amount of solvent. For example, about 40 times of methanol are used relative to the sucrose fatty acid ester in the previously cited Rizzi et al. Patent. This is, of course, uneconomical and requires a large amount of investment for the solvent recovery system and anti-explosion facilities. Additionally, certain amounts of sucrose fatty acid esters dissolving in the solvent are unavoidably wasted.
Sucrose fatty acid polyesters are known as suitable low-calorie fat replacers in edible products. Substantially indigestible by human beings, they have physical and organoleptic properties very similar to triglyceride oils and fats conventionally used in edible products as described, for example, in U.S. Pat. Nos. 3,600,186, 4,005,195 and 4,034,083. In addition, U.S. Pat. No. 5,077,073 discloses ethoxylated sugar or sugar alcohol sucrose fatty acid esters useful as fat substitutes wherein from 1 to 50 alkoxyl groups are attached by ether linkages to each polyol molecule. However, the use of sucrose fatty acid polyesters which are liquid below body temperature (about 37.degree. C.) has been reported to result in an undesired laxative effect and give rise to the problem of anal leakage. Thus to overcome this problem, it has been proposed to introduce considerable amounts of solids in the sucrose polyester phase by adding solid fatty acids, or employing a polyester which is partially liquid and partially solid at body temperature.
It is accordingly a main object of this invention to provide a process for producing sucrose fatty acid polyesters in an efficient manner which is free from the above-described disadvantages and which is simple in operation and easy to control.